Fix the zero page allocator

Since the underlying kmem allocator uses the first sizeof(struct run) bytes as metadata for freed runs, the zero pages put into the zero page allocator's kmem were coming back with the first few bytes non-zero. Fix this by explicitly zero'ing the run header of pages retrieved from kmem. usertests now passes the bss test. It still fails twofiles and forktest.

Fix the zero page allocator
6f540f24 · Austin Clements · a184838e · 6f540f24
--- a/kernel/zalloc.cc
+++ b/kernel/zalloc.cc
@@ -80,6 +80,9 @@ zallocator::alloc(const char* name)
    p = kalloc(name);
    if (p != nullptr)
      zpage(p);
+  } else {
+    // Zero the run header used by kmem
+    memset(p, 0, sizeof(struct run));
  }
  tryrefill();
  return p;